This addresses the enhancement requested in: https://github.com/ArduPilot/ardupilot/issues/2424
It checks various PID values to check if they are zero, if they are and the corresponding rate forward feed is zero then it flags it as PreArm failure.
It has been added to the parameter check, so can be overridden if required.
Stop "latching" calibration checks - if an RC radio's calibration
changes after it passes once, these patches allow the rc calibraiton
checks to then fail.